[memprof] Add Version2 of IndexedMemProfRecord serialization (#87455)

I'm currently developing a new version of the indexed memprof format
where we deduplicate call stacks in IndexedAllocationInfo::CallStack
and IndexedMemProfRecord::CallSites.  We refer to call stacks with
integer IDs, namely CallStackId, just as we refer to Frame with
FrameId.  The deduplication will cut down the profile file size by 80%
in a large memprof file of mine.

As a step toward the goal, this patch teaches
IndexedMemProfRecord::{serialize,deserialize} to speak Version2.  A
subsequent patch will add Version2 support to llvm-profdata.

The essense of the patch is to replace the serialization of a call
stack, a vector of FrameIDs, with that of a CallStackId.  That is:

  const IndexedAllocationInfo &N = ...;
  ...
  LE.write<uint64_t>(N.CallStack.size());
  for (const FrameId &Id : N.CallStack)
    LE.write<FrameId>(Id);

becomes:

  LE.write<CallStackId>(N.CSId);

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/ProfileData/InstrProfReader.cpp b/llvm/lib/ProfileData/InstrProfReader.cpp
index 7ac5c56..884334e 100644
--- a/llvm/lib/ProfileData/InstrProfReader.cpp
+++ b/llvm/lib/ProfileData/InstrProfReader.cpp
@@ -1303,7 +1303,7 @@ Error IndexedInstrProfReader::readHeader() {
     MemProfRecordTable.reset(MemProfRecordHashTable::Create(
         /*Buckets=*/Start + RecordTableOffset,
         /*Payload=*/Ptr,
-        /*Base=*/Start, memprof::RecordLookupTrait(Schema)));
+        /*Base=*/Start, memprof::RecordLookupTrait(memprof::Version1, Schema)));
 
     // Initialize the frame table reader with the payload and bucket offsets.
     MemProfFrameTable.reset(MemProfFrameHashTable::Create(